DE19911923A1 - Ferroelectric liquid crystal display with active matrix elements and asymmetrical orientation layers - Google Patents

Abstract

In the monostable chiral-smectic active matrix display, which contains a liquid crystal layer between two substrates connected to the outside with electrodes, the layers adjacent to the liquid crystal layer or the layer systems of the two substrates between the liquid crystal layer and electrodes differ in at least one physical and / or chemical property .

Description

The replacement of the cathode ray tube (picture tube) with a flat screen requires a display technology that simultaneously has a high resolution, ie more than 1000 lines, a high brightness (<200 Cd / m ² ), a high contrast (<100: 1), a high frame rate (<60 Hz), sufficient color display (<16 million), large image format (<40 cm), low power consumption and a wide viewing angle, combined with low manufacturing costs. There is currently no technology that fully fulfills all of these features at the same time.

Many manufacturers have developed screens based on nematic liquid crystals, which have been in use in the area of notebook PCs, personal digital assistants, desktop monitors, etc. for several years. The technologies STN (Supertwisted Nematics), AM-TN (Active Matrix-Twisted Nematics), AM-IPS (Active Matrix-In Plane Switching), AM-MVA (Active Matrix-Multidomain Vertically Aligned) are used in the relevant literature are described in detail (see e.g. BT Tsukuda, TFT / LCD:
Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach 1996, ISBN 2-919875-01-9 and literature cited therein; SID Symposium 1997, ISSN-0097-966X and literature cited therein). In addition, reference is made to the technologies PDP (Plasma Display Panel), PALC (Plasma Addressed Liquid Crystal), ELD (Electro Luminescent Display), FED (Field Emission Display) etc., which are also explained in the SID report cited above.

Clark and Lagerwall (US 4,367,924) were able to show that the use ferroelectric liquid crystals (FLC) in very thin cells to optoelectric Switching or display elements leads, compared to the conventional TN ("twisted nematic") - cells up to a factor of 1000 faster switching times have (see e.g. EP-A 0 032 362). Because of this and others cheaper Properties, e.g. B. the bistable switching option and almost angle-independent contrast, FLCs are basically for Areas of application such as computer displays and televisions suitable as a Monitor marketed by Canon in Japan since May 1995.

For the use of chiral smectic liquid crystals in electro-optical or completely optical components you need either connections, which develop smectic phases and are themselves optically active, or you can by doping compounds that have such smectic phases train, but are not optically active themselves, with optically active connections induce ferroelectric smectic phases. The desired phase should be be stable over the largest possible temperature range.

A smectic liquid crystal display is made of polarizers, with electrodes, e.g. B. from indium tin oxide, coated substrates made of plastic or glass, which are provided with one or more orientation layers and possibly further layers (passivation, diffusion barrier, insulation, antireflection layers, etc.) and color filter layers, a liquid crystal layer and possibly active thin-film elements built up. (see e.g. BC Prince, Seminar Lecture Notes, Volume I, pages M-3/3-M-3/22, SID International Symposium 1997, Boston, USA; BB Bahadur, Liquid Crystals Application and Uses, Vol. 1, page 410 ff, World Scientific Publishing, 1990; E. Lüder, Recent Progress of AMLCD's, Proceedings of the 15 ^th international display research conference, 1995, pages - 12).

The individual picture elements (pixels) of an LC display are usually in one x, y matrix arranged by the arrangement of a series of electrodes (Conductor tracks) along the rows and the columns on the lower or Top of the display is formed. The crossing points of the horizontal (Row) and vertical (column) electrodes form addressable pixels.

This arrangement of the pixels is usually referred to as a passive matrix. Various multiplexing schemes have been developed for addressing, as described, for example, in Displays 1993, Vol. 14, No. 2, pp. 86-93 and contacts 1993 (2), pp. 3-14. Passive matrix addressing has the advantage of simpler manufacture of the display and the associated low manufacturing costs, but the disadvantage that passive addressing can only be carried out line by line, which means that the addressing time of the entire screen for N lines is N times the line addressing time is too slow for moving pictures. The display of grayscale is also difficult. Mizutani et.al. Conference FLC in Brest, France have on the (20 to 24 May 1997, see Abstract Book 6 ^th International Conference on Ferroelectric Liquid Crystals, Brest / France) passive FLC display unveiled a digital grayscale. The disadvantage of this method is a large increase in the number of display drivers required and thus in the cost.

In the so-called active matrix technology (AMLCD) is usually a non-structured substrate combined with an active matrix substrate. On everyone Pixel of the active matrix substrate is an electrically non-linear element, for example, a thin film transistor integrated. With the non-linear Element can also be diodes, metal insulator metal and the like. elements  act, which are advantageously produced with thin film processes and in the relevant literature are described (see e.g. T. Tsukuda, TFT / LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach 1996, ISBN 2-919875-01-9 and literature cited therein).

Active matrix LCDs are usually built with nematic liquid crystals TN- (twisted nematics), ECB- (electrically controlled birefringence), VA- (vertically aligned) or IPS (in plane switching) mode. In any case the active matrix creates an electric field at every pixel individual strength that creates an orientation change and thus a Change in birefringence is generated, which in turn is in polarized light is optically visible. A serious disadvantage of these methods is that lack of video ability, d. H. the too long switching times nematic Liquid crystals.

For this reason, among other things, liquid crystal displays based on the Combination of ferroelectric liquid crystal materials and active matrix Elements based, e.g. B. in WO 97/12355 or in Ferroelectrics 1996, 179, 141- 152 or from W. J. A. M. Hartmann (IEEE Trans. Electron. Devices 1989, 36, (9; Pt. 1), 1895-9, and dissertation Eindhoven, Netherlands 1990).

Hartmann used a combination of the so-called "quasi-bookshelf Geometry "(QBG) from FLC and a TFT (thin film transistor) active matrix and got high switching speed, grayscale and high at the same time Transmission. However, the QBG is not over a wide temperature range stable, because of the temperature dependence of the smectic layer thickness field-induced layer structure breaks open or rotates.  

Nito et. al. have proposed a monostable FLC geometry (Journal of the SID, 1/2, 1993, pages 163-169), in which the FLC material with the help relatively high voltages is oriented such that only a stable Position arises from which then when an electric field is applied over a Thin film transistor generates a number of intermediate states, which at adapted cell geometry between crossed polarizers of a series of correspond to different degrees of brightness (gray values).

The disadvantage of working with Nito et.al. is now the appearance of a stripe texture that limits the contrast and the brightness of this cell (see Fig. 8 of the above quotation). The disadvantageous stripe texture can be corrected by treatment with a high electric field (20-50 V) in the nematic or cholesteric phase (see p. 168 of the above quotation); however, such a field treatment is not suitable for the mass production of screens and generally does not lead to temperature-stable textures. In addition, this method only results in switching in an angular range of up to a maximum of the simple tilt angle, which in the case of Nito et. al. The material used is approx. 22 ° (see p. 165 Fig. 6) and thus only results in a transmission of a maximum of 50% of the transmission of two parallel polarizers.

For the representation of gray values or of as many natural ones as possible The characteristic curve (transmission versus voltage) should be colors on the one hand be flat enough to be able to target the gray values with the available voltages to address, on the other hand, the saturation voltage for 90% of the Maximum transmission (effective tilt angle = 45 °) should not be too high, preferably in the range below five volts (5 V), in particular below three volts (3 V). It is desirable to have the characteristic for a given To be able to influence liquid crystal mixture so that it is flat enough to addressing all gray values with the available voltages, and does not have too high a saturation voltage for 90% of the maximum transmission.  

The object of the present invention is to provide an active matrix Displays that influence the characteristic of a given Liquid crystal mixture allowed in an advantageous manner.

It has been found according to the invention that the characteristic curve for a Liquid crystal layer in a monostable chiral smectic active matrix Display can be affected if the on the liquid crystal layer adjacent layers of the two substrates in at least one physical and / or differentiate chemical property from each other.

The invention thus relates to a monostable chiral-smectic active matrix Display that is between two substrates connected to the outside with electrodes Contains liquid crystal layer, preferably in the form of a monodomain, wherein the layers adjacent to the liquid crystal layer or those between Liquid crystal layer and electrodes located layer systems of the two Substrates with at least one physical or chemical property differentiate from each other.

The active matrix displays according to the invention are generally made of Polarizers made of substrates coated with electrodes such as indium tin oxide Plastic or glass with one or more orientation layers and possibly other layers (such as passivation, diffusion barrier, Insulation anti-reflective layers, etc.) and color filter layers and a liquid crystal layer that lies between the substrates, built up. In addition, the displays can have active thin-film elements. For the structure of the active matrix displays, reference can be made to the literature cited at the beginning to get expelled. Under the expression "those on the liquid crystal layer adjacent layers of the substrates "are in the presence of Orientation layers understood these orientation layers. If no  such layers exist, it is the top of the Layer of substrates facing liquid crystal layer. It is therefore about the "inner" layers of the substrates, which are in contact with the liquid crystal layer To be in touch. It is irrelevant whether additional passivation, Diffusion barrier, insulation, anti-reflective layers or similar layers possibly still present on the surface. Even if there are Layers become the orientation layer or the inner layer of the Substrates viewed as "layer adjacent to the liquid crystal layer".

Due to the different physical and / or chemical properties of the layers adjacent to the liquid crystal layer has a picture element (pixel = picture element) of the monostable chiral smectic according to the invention Displays, apart from marginal areas, an asymmetrical cross section in vertical direction, that is perpendicular with respect to the Liquid crystal layer. The asymmetry can affect the geometric Dimensions or relations of those applied to the liquid crystal layer Relate layers or layer systems, as well as the dielectric Properties or for example surface properties such as Surface tensions or surface anisotropies. By the at least one different physical or chemical property of the layers that an asymmetry is generated between the liquid crystal and the electrodes.

The ones lying on the liquid crystal layer preferably differ Layers of substrates in the orientation. According to one Embodiment of the invention only one applied to the liquid crystal layer Layer an orientation layer. According to a further embodiment According to the invention, both layers adjoining the liquid crystal layer Orientation layers that differ in orientation or it the surface roughness and / or the surface tilt angle can differ his. Specifically, e.g. B. the transparent conductor track by etching processes be roughened differently and thus the orientation effect  be influenced asymmetrically. The orientation layers can for example, differ in the strength and / or direction of orientation.

The characteristic curve is advantageously influenced via the Selection of suitable orientation layers.

The invention preferably relates to a monostable active matrix Display, in which the targeted combination of orientation layers an optimal characteristic is obtained.

It was found that the characteristic curve tends towards lower voltages shifts when the two 'top' and 'bottom' to the liquid crystal layer adjacent layers or layer systems extending to the electrode differentiate from each other. This applies in particular to the use of two different orientation layers, above and below the Adjacent the liquid crystal layer. These layers are also express then different if they are made of the same material, but different were pretreated, e.g. B. one side rubbed hard, the other side weak or was not rubbed at all. Cells with only one are also suitable Orientation layer.

In the monostable chiral smectic active matrix Displays can be any suitable chiral-smectic, in particular ferroelectric liquid crystal mixtures are used. The Liquid crystal mixture takes on a monostable position, but forms in the process no stripe texture, is temperature stable and enables a very high Maximum transmission and a very high contrast.  

The active matrix FLC display according to the invention preferably contains, as an optically active layer, a ferroelectric liquid-crystalline medium with a phase sequence of
Isotropic-nematic or cholesteric-smectic C *
or the phase sequence
Isotropic-nematic or cholesteric-smectic A-smectic C *,
in this case, however, characterized in that the smectic A phase has a range of existence of at most 2 ° C, preferably at most 1 ° C.

The following examples are intended to explain the invention in greater detail, but without it to limit it.

Examples example 1

An LCD test cell is placed on two commercially available, with indium tin oxide made of conductive transparent coated glass plates. These are with the Orientation layer LQT-120 (manufacturer: Hitachi Chemicals KK), which with N- Methylpyrrolidone is diluted to 8.3% of its original solids content, coated by spin coating (2500 U / min. 10 sec), hardened by heating (230 ° C, 1 hour) and then a rubbing process for orientation subjected (rubbing material: rayon type YA-20-R *, clearance 0.2 mm, 1 time, 700 U / min roller speed, 10 cm / s substrate speed, 10 cm Roll diameter). The friction pressure can be adjusted via the clearance  Distance between friction material and substrate) can be varied and is considered 'strong' or 'weak'.

The rubbed glasses are glued with the rubbing direction parallel to the test cells and adjusted to a distance of 1.3 µm using a spacer. Three types of cells are made:

• a) symmetrical with weak rubbing pressure on the upper and lower Glass plate (cell 1)
• b) symmetrical with strong rubbing pressure on the upper and lower side (Cell 2)
• c) unbalanced with strong pressure on one and weak pressure on the other side (cell 3)

The cells are now each chiral-smectic test mixture of the phase sequence

X <0 Sc * 78.2 N * 103.0-105.7 I

filled and by cooling to <100 ° C initially in the nematic phase oriented. In the temperature range from 80 ° C to 70 ° C, the test mixture is used a cooling rate of 1 ° C / minute with an applied DC voltage of 3 V. oriented and then cooled to the examination temperature of 25 ° C. Here a monodomain is obtained, which is in an electro-optical measuring apparatus their electro-optical properties are examined.

The following transmission values * are obtained as a function of the applied voltage.

* Transmission based on two parallel polarizers, i.e. H. 100% corresponds to this experiment of the theoretical maximum transmission of the cell.

The example shows the advantageous shift in the threshold voltage smaller values when using asymmetrical cells.  

Example 2

An LCD test cell is made as described in Example 1, but is coated only one of the two substrates, but without the second substrate Orientation layer rubbed (cell a). For comparison, there is also one symmetrical cell, i. H. both sides coated with LQT120, manufactured (cell b).

The rubbed glasses become the rubbing direction with parallel alignment Test cells glued and using a spacer to a distance of 1.3 µm set.

The cells are now each with a chiral-smectic test mixture of spontaneous polarization 4 nC / cm ² and the phase sequence

X <0 Sc * 67.6 N * 91.1-93.9 I

filled and by cooling to <90 ° C initially in the nematic phase oriented. In the temperature range from 70 ° C to 60 ° C, the test mixture is used a cooling rate of 1 ° C / minute with an applied DC voltage of 3 V. oriented and then cooled to the examination temperature of 25 ° C. Here a monodomain is obtained, which is in an electro-optical measuring apparatus their electro-optical properties are examined.

The following transmission values * are obtained as a function of the applied voltage.

* Transmission based on two parallel polarizers, i.e. H. 100% corresponds to this experiment of the theoretical maximum transmission of the cell.

The example also demonstrates the advantageous shift in the threshold voltage to smaller values when using asymmetrical cells.  

Example 3

An LCD test cell is made as described in Example 1, but is coated only one of the two substrates, but without the second substrate Orientation layer rubbed (cell a). For comparison, there is also one symmetrical cell, i. H. both sides coated with LQT120, manufactured (cell b).

The rubbed glasses become the rubbing direction with parallel alignment Test cells glued and using a spacer to a distance of 1.3 µm set.

The cells are now each with a chiral smectic test mixture of spontaneous polarization 4 nC / cm ² and the phase sequence

X <0 Sc * 63.1 N * 89.6-92.1 I

filled and by cooling to <90 ° C initially in the nematic phase oriented. In the temperature range from 66 ° C to 58 ° C, the test mixture is used a cooling rate of 1 ° C / minute with an applied DC voltage of 3 V. oriented and then cooled to the examination temperature of 25 ° C. Here a monodomain is obtained, which is in an electro-optical measuring apparatus their electro-optical properties are examined.

The following transmission values * are obtained as a function of the applied voltage.

* Transmission based on two parallel polarizers, i.e. H. 100% corresponds to this experiment of the theoretical maximum transmission of the cell.

The example also demonstrates the advantageous shift in the threshold voltage to smaller values when using asymmetrical cells.

Claims (8)

1. Monostable chiral-smectic active matrix display, which contains a liquid crystal layer between two substrates connected to the outside with electrodes, characterized in that the layers adjacent to the liquid crystal layer or the layer systems of the two substrates located between liquid crystal layer and electrodes in at least one physical and / or chemical property from each other. 2. Active matrix display according to claim 1, characterized in that the layers of the substrates in contact with the liquid crystal layer in the Distinguish orientation effect. 3. Active matrix display according to claim 1 or 2, characterized in that only one of the substrates as a layer adjacent to the liquid crystal layer has an orientation layer. 4. active matrix display according to claim 1 or 2, characterized in that both substrates as layers adjacent to the liquid crystal layer Have orientation layers that differ in the orientation effect differentiate. 5. active matrix display according to claim 4, characterized in that the Orientation layers in the strength and / or direction of orientation differentiate.   6. active matrix display according to claim 1, 2 or 4, characterized in that the layers adjacent to the liquid crystal layer in the Distinguish surface roughness and / or in the surface tilt angle. 7. Active matrix display according to one of claims 1 to 6, characterized characterized in that the liquid crystal layer is in a monodomain. 8. Active matrix display according to one of claims 1 to 7, characterized in that the liquid crystal layer is chiral-smectic and the phase sequence
IN-smC * or
IN-smA (with an area of existence <2 ° C) -smC *.